I actually had a problem like this, but it started for no apparent reason. Antibiotics reduced it a bit, but once the course was finished, it just stayed that way. The snake was eating again. It took almost a year and a half for the snake's face to look mostly normal again. The only thing a vet can do is surgery to remove the pus (which in reptiles forms a semi-sold gel rather than a liquid), and scans to make sure it's not cancer or a bone infection. The surgery didn't seem like the best option to me--seems like it has more risks than just waiting.
I would have it cultured, and if nothing really comes up, and the snake is eating...observe and see what happens. It LOOKS reddened, so there may be an active infection going on there, and that needs to be treated promptly. Just don't be surprised if eliminating the infection doesn't eliminate the swelling.